Add generateTestHeaderStringAsync function to Webhooks.ts

[zelief]

Currently, generateTestHeaderString throws a CryptoProviderOnlySupportsAsyncError when executed in Bun and other non-Node environments. This presents a limitation for users attempting to utilize this function outside of Node.js.

This PR addresses this issue by adding generateTestHeaderStringAsync. With this, creating a test header string in non-Node environments becomes more seamless and straightforward.

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[ramya-stripe]

Hey @zelief

There is some duplicated code between generateTestHeaderString and generateTestHeaderStringAsync. Can you consider sharing code between the two to ensure they dont diverge in the future?

[zelief]

Hey @zelief

There is some duplicated code between generateTestHeaderString and generateTestHeaderStringAsync. Can you consider sharing code between the two to ensure they dont diverge in the future?

Sure @ramya-stripe, I will update it. Thanks for the review!

[zelief]

Updated @ramya-stripe, kindly check.